petrology and geochemistry ofash shafa plutonic rocks · 2009. 4. 18. · geochemistry of the...

J KA U: Earth sa.,vol. 1, pp. 149-163 (1408A .H.l1988A .D.)

Petrology and Geochemistry of Ash Shafa Plutonic Rocks

F.M. MARZOllKl" ' and H.A. QURASHlI!'

Faculty of Earth Sciences, King Abdulaziz University'!',and

Directorate General of Mineral Resources'?',Jeddah, Saudi Arabia.

ABSTRACT. Ash Shafa area is underlain by metamorphic rock s con sisting ofa mphibo lite schi st. quart zo-fcldsparhic gneiss a nd marble . These mct arnor­ph ic rocks were later invaded by different typ es of plutonic rock s includinggabbro-dio rite . tonalite-quartz diorite and gneissic granite-granite . Ch emi­cal data indic at e that Ash Shafa plutonic rocks have a calc-alkaline affinity .Th e gabbro -d ior ite-q uartz diorite-tonalite were formed in an ensimatic is­land-arc , whereas the granites were formed by continental colli sion-rel atedmagmatism .

Introduction

The purpose of this paper is to describe brief1y the regional geology. petrography andgeochemistry of the various plutonic rocks in the Ash Shafa area . The Ash Shafa arealies in the southern part of At-Taif quadrangle between latitudes 21°07' OO"N and21°I I '23"N and lungitudcs 40°21'21"E and 40°25'23" (Fig . 1). This paper utili zes 14chemical analyses from the various plutonic rocks in Ash Shafa area . The plutonicrock petrographic terminology follows Streckcisen (1976) and Ramsay et al. (1986b) .

Regional Geology

The tectonic setting model proposed by Stoeser and Camp (cf. Stoeser 1986) sub­divided the Saudi Arabian part of the Arabian Shield into five terranes separated byfour suture zones. Th e three terranes (Asir , Hijaz, Midyan) west of the Nabitah su­ture are of ensimatic character, whereas the eastern two terranes (Afif, Ar Rayn) areof continental-marginal to continental (Stoeser 1986). The Asir terrane. which in­cludes the studied area , can he divided into a number of tectonostratigraphic beltsthat contain assemblages of primitive to moderately evolved arc-type volcanoc1asticsedimentary rocks , basaltic and dacitic volcanic rocks, and dioritic to tonaliticplutonic rocks (Greenwood et al. 1982, Johnson and Vranas 1984).

The Ash Shafa area is underlain y rocks of the Baish , Samran , Hali and probablyJeddah Groups of the southe rn S tield (Andreason et al. 1977, and Smith and Irvine1980). Andreason et al. (1977) correlated the metamorphosed layered rocks which

149